This invention relates generally to packed absorption towers and, more particularly, to a packed absorption tower with an improved system for distributing fluid over the packing in the tower.
In so-called "gas absorption" operations, soluble portions of a gas mixture are dissolved in a liquid. Typically, gas absorption is conducted in a vertical gas/liquid contact zone such as a packed tower or tray tower. Packed towers are the most common.
A packed tower is filled with a bed of packing material which may be dumped randomly or arranged in "structured" fashion. The bed of packing is permeable to gas and liquid flow, and provides surface area for intimate gas/liquid contact and consequent mass transfer of soluble components of the gas to the liquid phase. In a countercurrent system (the most common type), the absorbing liquid is introduced into the top of the tower, and the rich gas is introduced into the bottom of the tower; enriched liquid exits the bottom of the tower, and lean gas exits the top of the tower.
For example, in the manufacture of sulfuric acid, a combustion gas containing sulfur dioxide is produced by burning sulfur in excess air, and sulfur dioxide in the combustion gas is converted to sulfur trioxide by passage of the gas over a catalyst. Sulfur trioxide is removed from this "converter" gas by absorption in sulfuric acid. Sulfuric acid, having a concentration of 98.3%, for example, is introduced into the top of the absorption tower and percolates downwardly through the packing. Sulfur trioxide laden gas enters the bottom of the tower and flows upwardly through the packing where it contacts the downward flowing acid. Sulfur trioxide is transferred from the gas to the liquid phase, reacting with water to form additional sulfuric acid. The acid exits the bottom of the tower at a concentration of, e.g., 99.2%. This acid stream is cooled in a heat exchanger (to remove substantial heat of absorption) and a portion of the cooled acid is removed from the system as product, while the bulk of the cooled acid is diluted with water back to 98.3% and recirculated to the top of the absorption tower. Gas leaving the top of the tower is directed to the stack or to other steps in the process.
To assure effective mass transfer, and especially to prevent channeling of sulfur trioxide laden gas through the tower without adequate contact with the liquid, it is important to assure that the acid entering the tower is evenly distributed over the packing. This way the packing is uniformly wetted with the downflowing liquid, and there is a uniform axial (vertical) acid concentration gradient, with little or no radial (horizontal) acid concentration gradient, throughout the tower. This provides maximum driving force for mass transfer and minimizes channeling.
The most traditional systems for distributing fluid over packing are pipe-type systems and trough-type systems, the latter of which comprise a series of troughs having downtubes (sometimes referred to as "downcomers") which deliver fluid from the troughs to the packing. Trough-type systems have many advantages over pipe-type systems, but they require a network of overhead piping to distribute fluid to the various troughs of the system. This network is expensive to manufacture and install.